1. Field of the Invention
The invention relates to a method and system for generating electric energy in a tire. The word “tire” is referred to as “tyre” in the remainder of the specification, and the terms are intended to be used interchangeably.
2. Description of the Related Art
The generation of electric energy in a tyre can be useful for feeding electronic devices associated with the tyre.
The incorporation of electronic devices inside tyres is having increasing importance in order to increase the safety of vehicles. Such devices can, for example, include sensors and other components adapted to obtain information concerning various magnitudes of a tyre like, for example, temperature, pressure, acceleration, number of revolutions of the tyre, speed of the vehicle or actuators, like pumps adapted to keep the pressure inside the tyre substantially constant. Such devices can also include a transmitter (typically wireless) to send outside the tyre (typically to an on board control unit of the vehicle) the information obtained through the sensors and a microprocessor adapted to collect and process the signals coming from the sensors, before transmission. Optionally, such devices can also include a receiver (typically wireless) to receive possible information from the outside (for example, from an on board control unit of the vehicle).
EP 1 700 351 describes a process and a system for generating electric energy inside a tyre wherein a housing including a piezoelectric element is coupled with a tyre portion at a tread area of the tyre. The piezoelectric element is arranged along a plane perpendicular to a radial direction of the tyre and has a first end fixed to the housing and a second end fixed to a load mass. A gap is formed between an inner wall of the housing and an outer surface of said load mass. The piezoelectric element, the load mass and the gap are sized so as to obtain: a) during the rotation of the tyre on a rolling surface at a first rotation speed below a given speed, an oscillation within said gap of the load mass fixed to the piezoelectric element; and b) during the rotation of the tyre on the rolling surface at a second rotation speed above said given speed, a contact of the load mass with the inner wall of the housing, during a first fraction of a complete tyre revolution, and an oscillation within the gap of the load mass fixed to the piezoelectric element, during a second fraction of a complete tyre revolution. During the first fraction of tyre revolution the tread area is not in contact with the rolling surface, whereas during the second fraction the tread area is in contact with the rolling surface.
US 2008/0264537 describes a tyre module to detect various parameters on the conditions of a tyre, comprising a spring element locked to one end and a conversion unit, wherein kinetic energy is converted into electric energy. In an inductive embodiment of the conversion unit, the module has a cylindrical symmetry with a permanent magnet placed inside it, along the axis of the cylinder. The ferromagnetic circuit closes by means of the walls of the cylinder. The spring element has a circular or concentric leaf design and is fixed onto a cover of the cylinder. A seismic mass is arranged at the centre of the leafed elastic element, at the axis of the cylinder. Depressions, preferably concentric, make the leafed elastic element oscillating. The space between the permanent magnet and the seismic mass varies due to the acceleration jumps during the passage along the contact area of the tyre with the road, thus varying the magnetic flux in the ferromagnetic circuit. The variation of magnetic flux can be converted into an electrical voltage by means of an appropriate winding. In another inductive embodiment of the conversion unit, the module comprises a spring element, a permanent magnet fixed to the free end of the spring element and a winding fixed onto the fixed end of the spring element.
U.S. Pat. No. 7,285,868 describes a technique for generating electric energy in a tyre that exploits the load induced by the deflection of the inner walls of the tyre at the shoulder, close to the tread. In a radial embodiment, the energy generator comprises a chamber mounted on the inner surface of the tread, a magnet fixed to the base of the chamber, a winding movable along a guide tube, a spring fixed to one of the two inner walls, a cable and rollers. When the tread at the generator reaches the contact region with the road, the inner walls of the tyre get squashed, the centrifugal force generated by the rotation of the tyre goes to zero and the spring-cable-rollers system pulls the winding towards the rim, generating a voltage pulse. When, on the other hand, the tread comes out from the contact region with the road, the inner walls of the tyre get stretched, the centrifugal force generated by the rotation of the tyre is restored and pushes the winding towards the inner surface of the tread, generating a second voltage pulse.
U.S. Pat. No. 7,285,868 also describes a horizontal embodiment in which the energy generator comprises a chamber mounted horizontally that is fixed through two attachment devices to the inner walls at the shoulder of the tyre. The chamber comprises a magnet movable inside a guide tube around which a winding is wound. The alternating movement of the inner walls, that get squashed within the contact region with the road and get stretched outside the contact region, causes a movement of the magnet with respect to the winding.
B. P. Mann and N. D. Sims (“Energy harvesting from the nonlinear oscillations of magnetic levitation”, Journal of sound and Vibration 319 (2009), 515-530) describe the results of an analysis carried out on the generation of electric energy on the basis of vibrations through a magnetic system comprising a teflon tube having two magnets at the two ends. The magnets are mechanically fixed to two respective threaded supports and their distance can be varied by making them slide along such supports. The system also comprises a central magnet positioned inside the tube between the other two magnets. The magnetic poles are oriented so as to repel the central magnet thus suspending it with a non-linear restoration force. Moreover, the system comprises an upper winding and a lower winding wound around the tube, respectively on an upper half and a lower half of the outer surface of the tube. The authors analyse the behaviour of the device under harmonic excitation.
WO 2008/062377 describes a device for converting into electric energy the mechanical energy associated with the vibrations generated by vibration means connected to the device. The device comprises at least one electromagnetic stator with a coaxial winding and a magnetic mass that is movable with respect to the stator. The stator, the winding and the magnetic mass form a magnetic circuit. The magnetic mass moves perpendicularly to the axis of the winding in response to vibrations generated by the vibration means, varying the distance between the stator and the magnetic mass. When the magnetic mass is far from the stator, the electromagnetic circuit is open and the overall reluctance of the magnetic circuit is high. When the magnetic mass is in contact with the stator, the magnetic circuit is closed and the overall reluctance of the magnetic circuit is low. Consequently, the position of the movable magnetic mass influences the overall reluctance of the magnetic circuit and therefore the value of the magnetic flux coupled on the winding.